Fibrous polyethylene materials

ABSTRACT

AN EXTENSIVELY FIBRILLATED THERMOPLASTIC RESIN MATERIAL AND METHOD FOR PREPARING THE SAME IS GIVEN WHEREBY A SOLID PARTICULATE THERMOPLASTIC RESIN WHICH IS INSOLUBLE IN A SELECTED LEACHING SOLVENT IS MIXED AND FIBRILLATED UNDER HEAT AND PRESSURE WITH ANOTHER THERMOPLASTIC RESIN WHICH IS SOLUBLE IN THE SELECTED LEACHING SOLVENT. FOLLOWING FIBRILLATION, THE RESIN MIXTURE IS SUBSEQUENTLY CONTACTED WITH THE SELECTED LEACHING SOLVENT TO REMOVE THE SOLUBLE RESIN THEREBY LEAVING AN EXTENSIVELY FIBRILLATED THERMOPLASTIC RESIN PRODUCT WHICH MAY BE USED AS A SYNTHETIC FABRIC, AND IN OTHER INDUSTRIAL APPLICATIONS.

Jan. 1, 1974 L. v. GALLAGHER 3,733,093 FIBROUS POLYETHYLENE MATERIALS Filed May 17, 1971 6 Sheets-Sheet 1 INVENTOR LAWRENCE Va GALLAGHER A'IT 1974 v. GALLAGHER 3,783,093

FIBROUS POLYETHYLENE MATERIALS Filed May 17, 1971 6 Sheets-Sheet 2 INVENTOR LAWRENCE V. GALLAGHER BY y u- ATTORNEY Jlll- 1974 v. GALLAGHER 3,783,093

FIBROUS POLYETHYLENE MATERIALS Filed llay 17, 1971 e Sheets-Sheet s FIG. 5

v INVENT OR LAWRENCE V, GALLAGHER AanMA ATTORNEY Jan- 1, 7 L. v. GALLAGHER FIBROUS POLYETHYLENE MATERIALS 6 Sheets-Sheet 4 Filed May 17, 1971 FIG FIG

ENVENTOR GALLACHER N.

LAWRENCE V YA ATTORNEY 1974 v. GALLAGHER FIBROUS POLYETHYLENE MATERIALS 6 Sheets-Sheet 5 Filed May 17, 1971 FIG FIG. 7A

INVENTOR LAWRENCE V. 29

ATTORNEY Jan. 1, 1974 L. v. GALLAGHER 3,733,093

FIBROUS POLYETHYLENE MATERIALS Filed May 17, 1971 6 Sheets-Sheet 6 INVENTOR LAWRENCE V. GALLAGHER United States Patent FIBROUS POLYETHYLENE MATERIALS Lawrence Vincent Gallacher, Norwalk, Conn., assignor to American Cyanamid Company, Stamford, Conn. Continuation-impart of abandoned application Ser. No. 820,761, May 1, 1969. This application May '17, 1971,

Ser. No. 144,410

Int. Cl. 1332b 3/26, 5/08 US. Cl. 161-170 6 Claims ABSTRACT OF THE DISCLOSURE This application is a continuation-in-part of my prior application, Ser. No. 820,761, filed May 1, 1969, now abandoned.

This invention relates to new and novel synthetic materials and a method for preparing the same. More particularly, it relates to novel extensively fibrillated synthetic material of unique structure.

Thermoplastic materials are known and used in many different and unique forms. One such form having many valuable uses is the pore-type which includes particulate and fibrous pore formers. In the current technology several different types of methods may be employed if this form of final thermoplastic product is desired.

One such method employs the mixing of a thermoplastic resin with the pore forming material followed by the subsequent removal of the pore forming material leaving a porous resin product. Another method involves the pressing and sintering of thermoplastic resin particles to form micropores in the interstices between the particles. Yet another method provides for the dispersing of an inert matrix material which is compacted and densified with the thermoplastic resin during a sintering operation and subsequently extracted thus leaving the porous thermoplastic material. Unfortunately the final product produced by many of these prior art methods is generally poor in quality and has low mechanical strength and low pore volume characteristics in addition to poor uniformity of product.

It is, therefore, an object of this invention to provide a new and novel material. A prime object is to provide a new and novel fibrous material which has good mechanical strength and handling characteristics. Another object is to provide a novel method for producing the new extensively fibrillated material. Yet another object is to provide a novel product and process for making an extensivly fibrillated thermoplastic fabric or leather-like mat or web-type structure which has many uses. Additional objects and advantages will appear to those skilled in the art from the ensuing specification and examples.

The present invention is concerned with the addition of a primary solid particulate thermoplastic resin which is characterized by being insoluble in a selected leaching agent or solvent to a secondary thermoplastic resin which serves as a dispersing matrix and is characterized by being soluble in the selected leaching agent or solvent. It has been surprisingly found that if these resins are or just slightly below the melting point of the primary "ice resin, a fibrous or fibrillated mat or web-type structure is formed by the primary resin. If the material is compressed, a higher degree of fibrillation occurs in the structure with enhanced properties and improved permeability. This fibrillated structure of the primary resin in the secondary resin matrix can then be leached with the selective solvent which removes the secondary resin leaving behind the extensively fibrillated or webbed type structure.

The novel extensively fibrillated or webbed type structure of this invention may more precisely be characterized as having the form of a nearly continuous and integral three dimensional network or structure containing interconnected ribbons, branching ribbons, or ligaments. The novel ribbon-like structures of this invention have their principal surfaces arranged parallel to the plane of the sheet. The thickness of the ribbon-like elements range from about 0.3 to about 20 microns and preferably in the range of 1 to 9 microns. The widths are not as well defined because of lateral branching, but generally they are in the range of about 1.0 to about microns and preferably they are from about 2 to about 60 microns. In many cases the ribbons and ligaments continue nearly the whole length of the sheet. Generally they are at least one inch in length and preferably at least 200 microns. Within the fibrillar structure itself are void areas between the ribbons which are substantially uniformly dispersed throughout the material with major dimensions essentially parallel to the sheet. The density of the sheet ranges from about 0.25 to about 0.6 and preferably from about 0.3 to about 0.5. Average void thickness which is the smallest dimension of the void, is approximately one micron with a range from about zero to about 25 microns. This structure is essentially the same for all semicrystalline thermoplastic polymers such as polyolefins, polyesters, polyamides and others when fibrillated according to this invention. FIGS. 1 thru 4 of fibrillated isotactic polypropylene and FIGS. 6A, 6B and 60 of fibrillated poly(1-butene) shown at increasing magnifications are representative of the type structure found from this invention. Similar structures are achieved when two or more semi-crystalline thermoplastic polymers are fibrillated.

More specifically, a fiber-forming semi-crystalline thermoplastic resin such as, for example, polyethylene, polypropylene, poly(1-butene), poly(4-methyl pentene), Poly- (glycolic acid), poly(ethylene terephthalate), poly(hexamethylene sebacamide), other polyolefins, polyesters, or polyamides, copolymers containing these polymers or mixtures of these or other semi-crystalline thermoplastic polymers is dispersed in a resin matrix such as, for example, poly(methyl methacrylate), polystyrene, polyisobutylene, poly(vinyl acetate), poly(ethylene oxide) or mixtures of these, and others, either plasticized or unplasticized, at a temperature above the melting point of the thermoplastic resin. The secondary resin is conveniently selected on the basis of its solubility in a selected solvent as opposed to the insolubility of the fiber-forming semicrystalline thermoplastic resin the same select solvent and the workable temperature of the secondary resin. It is important that the secondary resin be workable at the temperature selected and comparable in melt viscosity to the fiber-forming component at the working temperature. Thus, for example, if polyethylene is selected as the fiber-forming semi-crystalline thermoplastic resin and poly(methyl methacrylate) is selected as the secondary resin forming the resin matrix, the working temperature of the dispersion should be from about C. to about 275 C. but preferably from C. to about 210 C.

Further, the chemical natures of the matrix and fibrillator are important, for the interactions of the two phases at their interface depend upon them. It is for this reason,

it is believed, that polystyrene and poly (methyl methacrylate) matrices impart different properties to poly(tl-butene), and polyolefin fibrillator materials are improved when the poly(methyl methacrylate) matrix is modified with poly(ethylene oxide). Surface forces tend to relax the ribbon-like structural elements toward rod like, round cross-sections in opposition to viscous forces and, therefore, it is desirable to keep viscosities high and surface energy low. Modification of poly(methyl methacrylate) with poly(ethylene oxide) appears to produce low surface energies with polyolefins and a number of other polymers.

Dispersion, mixing, and blending may be accomplished on a twin-screw extruder, two-roll mill or other conventional high shear mixing device. Once the resin blend is uniform and the mixing advanced, continued shear will cause orientation and fibrillation of the thermoplastic resin. At this point the temperature may be maintained at the original level or may be lowered to increase the orientation and fibrillation of the primary thermoplastic resin. Alternatively, after mixing of complete, the resinmatrix mixture can be compressed, extruded, or treated in any other conventional way to achieve shear within the matrix thus causing the orientation and fibrillation of the thermoplastic resin. Such subsequent treatment after high shear mixing may enhance the desired orientation and fibrillation.

Subsequently, the prepared resin matrix mixture is treated with an appropriate leaching solvent such as, for example, toluene, acetone, ethylene dichloride, methylene chloride, methyl alcohol, or other appropriately selected solvent or solvent mixture which extract, leach, or dissolve the secondary resin or resin matrix thus leaving the fibrillated product. This extraction may be carried out by any conventional manner such as soaking or spraying and may be accelerated by heat, providing, however, that care be taken to maintain the fibrillated structure.

The fibrous mat or web-type structure of this invention does not contain extruded filaments or uniform pores but rather contains and is made up of a web of oriented, interconnected, directional fiber-like strands, membranes, ribbons, branched ribbons, and fibrils. This structure is shown by the photomicrographs of FIGS. 1 through where the surface of an extensively fibrillated isotactic polypropylene sheet is shown at a magnification of 50X in FIG. 1, 100x in FIG. 2, 300x in FIG. 3, and 1000 in FIG. 4. FIG. 5 is a 300x view of a fracture surface of the same material brittle fractured in a plane perpendicular to the direction of shear, illustrating the ribbonlike nature of the fibrillated material. All of these micrographs were made using conventional scanning electron microscopy on fully extracted materials.

Microscopic examination of the sheet reveals a network of micro-ribbons, ligaments, and fibrils. For example, FIG. 3 is a micrograph of a typical cross-section of a polypropylene sheet which has been sectioned parallel to the flow direction. Here we are looking at the leading edges of the ribbons. In this case which is typical, the width of the ribbons ranges from about 1.0 to 40 microns. However, it can be easily ascertained that the typical ribbons have a width of about microns. In like manner the ribbon thickness ranges from about 0.3 to 20 microns, but it can be easily seen that they typically are about 8 microns. The vertical space between the ribbons averages about 1 to 2 microns and ranges from nearly zero to 20 microns.

While different materials and process variations may vary these values for ribbon width, thickness, pore size, etc., these are typical and representative generally of the structures found in this invention. Within the scope of this invention are ribbon, fibril, or ligament structures which have a width to thickness ratio from 2:1 to 11:1 although I prefer ranges of from about 3:1 to 8: 1.

FIG. 5 is a top view of the polypropylene structure and it shows that the lengths of the ribbons are typically greater than the dimensions of the microscopic field, i.e., 900 microns at x. In fact they are usually larger than one inch although this can vary greatly depending upon the material system and process. Preferably the length of these structures is at least 200 microns. The network is aligned principally in the flow direction. However, there are many internal connections between fibrils and ribbons so that the extracted sheet is quite coherent and strong.

The primary resin may be modified by incorporating minor amounts of polytetrafluoroethylene. If this is desired, I usually employ less than 30% polytetrafluoroethylene in the primary resin and preferably from about 1% to 15% based upon the weight of the primary resin. The polytetrafiuoroethylene structure has the form of a three dimensional network of very fine fibrils. This polytetrafluoroethylene sub-structure consists of fibrils which generally connect and penetrate the larger fibrillated primary resin ribbons. There is a great deal of entanglement, and the fibrils range from less than about 0.02 micron to about 0.2 micron in diameter with inter fiber spacings ranging from about 0 to about 3 microns. The polytetrafiuoroethylene sub-structure can contribute to the overall physical strength of the sheet.

It is believed that the unique structure of this invention, the extensively fibrillated web or mat-type structure, is a function of the transmission of applied stress and shear forces of the roll mill, compression, or extrusion and additionally the temperature of the resin-matrix mixture during fibrillation. Thus, if a resin-matrix mixture is processed on a high shear device, low temperature processing at temperatures from about 15 centigrade degrees below the melting point to about the melting point of the thermoplastic resin will produce highly oriented web mat-type fiber structures which have low permeability. Processing at a temperature to about 90 centigrade tlegrees above the melting point and as high as at least centigrade degrees will result in products of less fibrillation and higher permeability. Thus, as temperature increases, the fibrillation decreases and conversely, the permeability increases. It should be noted that tensile strength properties improve with the lowering of processing temperatures below the melting point of the thermoplastic resin. The orientation we claim here is primarily at the fibril level with relatively little molecular orientation except where fibrillation is induced at or below the melting point.

When viewing and characterizing the extracted primary resin fibrillar structure either with or without fibrillar or non-fibrillar modifiers, the ribbons or ligaments sometimes give the appearance of being discrete. In many instances, however, they are connected to either or both their vertical or lateral neighbors. Thus it is sometimes difiicult to determine the proper width. In general when giving precise measurements for this dimension, I have measured the large well defined masses and not considered small interconnects to be part of the, individual ribbons.

The extensively fibrillated and novel structure of this invention can be used as a synthetic fabric or leather, a porous substrate in batteries or fuel cells, a filter or other industrial membrane, in bandages and other medical uses, and additionally has many applications in molding and extrusion situations.

One novel use is disclosed in my copending application which is now U.S. Pat. No. 3,675,391, patented July 11, 1972 wherein water rejection and breathability occur simultaneously. Typically, a blend of nine parts by weight polyolefin, and optionally One part by weight polytetrafluoroethylene, and 27 parts by weight of modified poly- (methyl methacrylate) matrix is compounded above the melting point of the polyolefin and softening point of the modified poly(methyl methacrylate), and then extruded to form a sheet. This is treated with methylene chloride to extract the soluble matrix, leaving behind a soft sheet of fibrillated polyolefin and polytetrafluoroethylene if added.

Another unique feature of thisprocess is that nonleachable polymeric fillers composed of fibrous or particulate polyethylene terephthalate, polyacrylonitrile, polyurethanes and other polymers may be incorporated into the matrix in the dispersion step. Additionally other organic or inorganic fillers such as, for example, soluble organic dyes, coloring pigments, carbon black, calcium carbonate, iron oxide, and other fillers may be added. Although binders and other materials may be initially added to the mix before forming the fibered structure, the unique process and properties of the product of this invention are achieved without fillers, additional binders, or internal bonding of the fibers after extraction.

As mentioned above, before extraction the material can be extruded, laminated to fabrics, or shaped into useful forms which remains after the extraction step. However, the final formed product may also be subsequently treated, cut, formed, or worked in a manner such as not to destroy the fibrillated structure. Additionally, the final product can be used with or bonded to substrates, such as cloth, metals, plastics, and other useful materials.

In order that the present invention may be more fully understood, the following examples are set forth by way of illustration. No specific detail or enumeration contained therein should be construed as limitations on the present invention excepting so far that they appear in the appended claims. All parts and percentages are by weight unless otherwise as specifically designated.

EXAMPLE 1 Sixty parts of molding grade poly(methyl methacrylate) pellets, twenty parts of powdered high molecular weight poly(ethylene oxide) and thirty parts of linear polyethylene pellets were blended in the same order on a two roll rubber mill with the rolls steam-heated to maintain the roll surfaces at 170 C. This temperature is well above the melting or softening points of each of the components, so that the mixture rapidly formed a visually uniform melt. After a uniform blend was obtained, milling was continued for another 5 minutes at the highest speed setting (50 r.p.m. on the faster roll, 36 r.p.m. on the slower one) to ensure intimate mixing of the components and to induce an orientation and partial fibrillation of the polyethylene. The blend was removed from the mill as a thick sheet and cut into plaques approximately 4 inches square. The plaques were pressed at 170 C., and then cooled under pressure to form sheets .035" thick. The sheetswere immersed at25 C. in a lightly agitated methylene chloride bath, which extracted substantially all (99%) of the poly(methyl methacrylate)/ poly(ethylene oxide) matrix within three hours.

The resulting product was a soft, White sheet with a leathery appearance. Electron microscopy revealed a structure of entangled fibers and ribbons similar to that in FIG. 3 with fiber diameters of a few microns. Water vapor transmission proved to be high: 1400 gram-mils/ 24 hours-100 inch using ASTM Method E-96, Procedure B. Permeability to air was measured with the Gurley densometer, and was very high: 40 seconds for the passage of 100 cc. of air through a 1 square inch area with a pressure differential of 0.013 atmosphere. Tensile properties were quite directional, with an average tensile strength of 200 p.s.i.

When the original plaques cut from the milled sheet were extracted in solvent without being pressed, and were subsequently examined microscopically, it was found that the primary structure consisted of oriented unidirectional ribbons and fibrils with much larger lateral dimensions than those in the pressed sheets. Thus, the pressing step results in an increase in the degree of fibrillation and in the entanglement of the fibrils.

The poly(ethylene oxide) in the formulation improves the blending of the polyethylene and poly(methyl methacrylate and plasticizes the poly(methyl methacrylate) so that it can be milled at lower temperatures. Further, sheets of the blend containing poly(ethylene oxide) can be handled and flexed easily at room temperature.

EXAMPLE 2 The same composition described in Example 1 was processed in the same way except that once a uniform blend was achieved on the mill at 170 C., the roll surfaces were allowed to cool below the melting point of the polyethylene, to C. while milling continued. The result was that the polyethylene began to crystallize on the mill while being sheared. Some of the plaques formed in this manner were pressed at 125 C. and extracted in methylene chloride to yield a somewhat stiffer sheet than that described in Example 1. The tensile strengths of the sheets were 2200 p.s.i. and 500 p.s.i. with and across the grain, respectively, and the passage time for 100 cc. of air was 800 seconds.

The rest of the plaques were pressed at C. and then extracted. The resulting sheets were quite similar in appearance and in physical properties to those of Example 1, indicating that the orientation effects achieved by milling at low temperatures were reversed by pressing at 170 C., 30 C. above the melting point of the polyethylene.

EXAMPLE 3 The composition described in Example 1 was preblended and pelletized using a twin-screw extruder. The blend was then extruded in a single-stage, single-screw extruder through a .030 x 4" sheet die, with a stock temperature in the die of 200 C. to produce a continuous sheet using a conventional sheet take-off system.

The sheet was extracted in methylene chloride as described in Example 1 to yield a soft, white sheet consisfing principally of highly fibrillated polyethylene with fiber diameters averaging 3-5 microns.

EXAMPLE 4 The sheet extruded in Example 3 was pressed at 170 C. to form a .035" sheet and subsequently extracted in methylene chloride, yielding a fibrillar sheet with a softer hand and higher permeability than the unpressed material.

EXAMPLE 5 .entangled fibrillar structure. The material possessed a tensile strength of 390 p.s.i., with no significant orientation effects, and was highly permeable to air, with an air fiow'time of 480 seconds.

EXAMPLE 6 When the components blended in Example 5 were processed in the same way, but using different proportions, the behavior and properties were found to vary systematically with composition. In general, as the polyethylene content was raised, the tensile strength and stiffness 'went up, the permeability decreased, and the time of extraction increased. The results are summarized in Table I below.

TABLE I Percent Flow time, polyethylene Tensile gurley in P E/PS strength, densorneter, blend Toluene extraction p.s.i. sec.

Disintegrated 20. Fast 390 480 35. do 580 608 50 Incomplete after 24 hours 1, 230

EXAMPLE 7 EXAMPLE 8 Sixty parts of molding grade poly(methyl methacrylate), twenty parts of high molecular weight poly(ethylene oxide), and thirty parts of isotactic poly(l-butene) were blended on a two roll mill at 170 C. for 10 minutes. The material was removed from the mill as a sheet and cut into 4 inch square plaques. These plaques were pressed into sheets at 170 C. and then extracted in an agitated methylene chloride bath. The product was an extremely soft, highly fibrillated sheet. The microstructure of this material is shown in FIGS. 6A, 6B, and 6C which were taken of the top surface using scanning electron microscopy at 50X, 300x, and 1000 respectively.

EXAMPLE 9 A blend of 60 parts of poly(methyl methacrylate), 20 parts high molecular weight poly(ethylene oxide), 3 parts poly(tetrafluoroethylene) in an aqueous dispersion and 27 parts of isotactic poly(l-butene), (all ingredients as in Example 8 except as noted) was made on a two roll mill at 170 C. The matrial was removed and cut into plaques as in the other examples, and these were pressed at 170 C. to form .035 inch sheets, cooled under pressure, and extracted in an agitated methylene chloride bath for two hours. This procedure removed over 99% of the poly- (methyl methacrylate) and poly(ethylene oxide), leaving an extremely soft, highly fibrillated sheet with the appearance and hand of fine glove leather. The tensile strength of this material was 450 p.s.i., the air flow time 550 seconds, and the water vapor transmission 1000 gram-mils/24 hours-100 in. Examination of this material using scanning electron microscopy revealed the structure shown in FIGS. 7A, 7B, and 7C, with a much finer fiber structure and a general enhancement of the fibrillation and entanglement caused by the poly(tetrafiuoroethylene). These figures are to be compared with FIGS. 6A, 6B, and 6C based on Example 8. The mechanism .for this enhancement is complex. The poly(tetrafluoroethylene) itself fibrillates in the matrix as a crystalline polymer via a cold-drawing response. Further, it is well known that poly(tetrafiuoroethylene) fibers greatly increase the melt elasticity and therefore the microscopic stress level in sheared polymer melts. Also, finely subdivided poly (tetrafluoroethylene) is an extremely effective nucleating agent for the crystallization of certain polyolefins. Contributions from each of these factors will combine to lead to the observed enhancement of the fibrillation of the principle fibrillator, in this case poly( l-butene). Similar effects have been noted in the fibrillation of each of the polyolefins named in these examples and in poly(ethylene terepthalate) as well. Therefore, the phenomenon is a general one and not restricted to the fibrillation of one or two polymers,

8 EXAMPLE 10 A blend with the same composition as in Example 9 was first blended and pelletized in a twin screw extruder. The pellets were then fed into a single stage 25 L/ D extruder with a uniform taper 2:1 compression screw and extruded through a .125 inch sheet die at 190 C. The extruded sheet was pressed to a thickness of .035 inch at 170 C. and this sheet was then laminated directly to a polyester fabric without adhesives by pressing at 170 C. After extraction with methylene chloride, the sheet was found to be substantially identical in appearance to the sheet produced in Example 9, except that it was now bonded directly to the polyester fabric and now possessed the tensile and tear properties of the fabric. Examination of the bond showed that the fibrillar polymer had actually penetrated and entangled with the fibers in the fabric to produce the bond.

EXAMPLE 11 Eighty parts of general purpose polystyrene, 3 parts poly(tetrafluoroethylene) and 27 parts of isotactic poly- (l-butene) were milled on a two roll mill at C. for 10 minutes. The blend was removed from the mill and cut into 4 inch square plaques which were then pressed at 130 C. to form sheets .035 inch thick. These were extracted in toluene at room temperature to yield highly fibrillar sheets somewhat stiffer than those described in Example 9. The tensile strength was 1300 p.s.i. and air flow time approximately 3700 seconds. Thus, the use of the polystyrene matrix instead of the poly(methyl methacrylate)/poly(ethyleneoxide) matrix resulted in a product with different characteristics.

EXAMPLE 12 A blend with the same composition as Example 11 was prepared on a two roll mill at 130 C. While milling continued, 27 parts of powdered calcium carbonate filler was gradually added and milled into the blend. Plaques were prepared and pressed at 130 C. to yield sheets, which were then extracted in toluene at room temperature. The sheets retained substantially all of the calcium carbonate after extraction, and further, it could not be removed or detected by rubbing or other surface contact.

Besides affecting the esthetic qualities of the extracted sheets, the filler had a profound effect on the speed of extraction and the physical properties as well. The extraction was accelerated in the presence of filler and this was paralleled by an increase in permeability to air. The physical properties are given in Table II below.

Seventy parts of a molding grade poly(methyl methacrylate), 10 parts of high molecular weight poly(ethylene oxide) and 30 parts of isotactic polypropylene all in pellet or powder form, were blended and pelletized in a twin screw extruder. The pellets were fed into a single stage 25/1 L/D extruder with a uniform taper screw having a 2:1 compression ratio. The blend was extruded through a sheet die with a .030 inch opening while the stock tem perature in the die was maintained at 210 C. The extrudate was extracted directly to yield a sheet of highly fibrillated polypropylene, with the bulk of the material consisting of small fibrils approximately 5 microns in diameter, mostly oriented in one direction, and ribbon-like structures in various stages of breaking down into fibrils.

A quantity of the extruded sheet was compressed to half its original thickness at (3., cooled under pressure, and extracted. The extracted sheet was softer than the original unpressed sheet, showed increased fibrillation on a microscopic scale, and higher permeability to air.

EXAMPLE 14 A blend of 1 part of isotactic poly(4-methyl pentene) and 3 parts of poly(methyl methacrylate) was prepared and pelletized using a twin screw extruder. The pellets were fed into a single screw extruder identical to that described in Example 13, with the stock temperature in the die maintained at 260 C. The .030 inch extruded sheet was extracted in methylene chloride for one hour to remove the poly(methyl methacrylate) matrix, leaving a soft, white, microfibrillar sheet of poly(4-methyl pentene). The fibrillar sheet was similar in appearance to the polypropylene sheet produced in Example 13.

EXAMPLE 15 A blend of 25 parts of poly(glycolic acid), and 75 parts of molding grade poly(methyl methacrylate) was preblended, pelletized, and extruded into sheet form exactly as described in Example 14. The product was extracted in acetone to dissolve the poly(methyl methacrylate) matrix, leaving a sheet of extensively fibrillated poly(glycolic acid). Examination of the product showed the structure to consist primarily of fibrillar semicontinuous rib bons, not completely broken down into the ultimate fibrils, but with the capacity to adsorb water readily.

EXAMPLE 16 A blend of 25 parts of poly(hexamethylene sebac amide) and 75 parts of molding grade poly(methyl methacrylate) was pro-blended, pelletized and extruded into sheet form exactly as described in Example 14. The product was extracted in methylene chloride, leaving a sheet of fibrillated poly(hexamethylene sebac amide) similar in appearance to the product in Example 15.

EXAMPLE 17 Eighty parts of pelletized molding grade poly(methyl methacrylate) was placed on a two roll mill maintained at 170 C. and milled until smooth. Thirty parts of the polyamide of i2-arninododecanoic acid pellets was gradually added while milling. Milling was continued for minutes after the blend appeared homogeneous in order to achieve maximum orientation and fibrillation of the polyamide. The blend was taken 011 the mill and cut into 4 inch square plaques. These were then pressed at 170 C. to form sheets, cooled, and extracted in methylene chloride to remove the matrix. The resulting sheet was tough and leathery, and consisted of highly fibrillated structure of the polyamide of l2- aminododecanoic acid.

EXAMPLE 18 Seventy parts of poly(methyl methacrylate) and 30 parts of pelletized poly(ethylene terephthalate) were blended and pelletized with a twin screw extruder. This blend was then extruded into sheet form exactly as described in Example 14. The product was extracted in acetone to yield a sheet of highly fibrillated poly(ethylene terephthalate) In the course of doing this work, it became apparent that the melt behavior of the matrix is important, in that the stress level imparted to the fibrillator component, and therefore the degree of melt orientation and fibrillation, is roughly proportional to the melt viscosity of the matrix. Thus, when the procedures performed in Examples 14 and were carried out with a lower molecular weight grade of poly(methyl methacrylate), with approximately 40% of the melt viscosity of the grade specified, the extracted product was much weaker and less desirable.

EXAMPLE l9 Sixty parts of poly(methyl methacrylate), 30 parts of high density polyethylene, and 20 parts of a thermoplastic polyurethane elastomer were blended on a two roll mill at 170 C. for 10 minutes. The material was removed from the mill and cut into plaques. These plaques were pressed into .015 inch sheets at 170 C. The sheets were soaked in an agitated methylene chloride bath at 75 F. to completely dissolve the poly(methyl methacrylate), leaving behind extremely tough, abrasion resistant sheets of highly fibrillated polyethylene and polyurethane elastomer.

While the foregoing invention has been described and exemplified in terms of its preferred embodiments, those skilled in the art will readily appreciate that variations can be made without departing from the sphere and scope of the invention.

I claim:

1. Fabric sheet consisting of a continuous integral network of interconnected ribbons and ligaments of thermoplastic resin comprising at least 60 percent by wt. of poly-alpha-olefin resin formed in a coherent flat web sheet with said interconnected ribbons and ligaments oriented longitudinally in said network parallel with the faces of said flat web and defining longitudinal voids between said ribbons and ligaments throughout said web,

- the dimensions of said ribbons and ligaments being in the ranges of one to microns width, 0.3 to 20 microns thickness and at least 200 microns length and the apparent thickness of said longitudinal voids being less than 25 microns.

2. A fabric sheet defined by claim 1 wherein the polyalpha-olefin resin consists of at least one member selected from the group consisting of: polyethylene, iso tactic polypropylene, isotactic poly(l-butene) and isotactic poly(4-methyl pentene).

3. A fabric sheet defined by claim 1 wherein the ratio of ribbon width to thickness is in the range from about 2. to 1 to about 11 to 1.

4. A fabric sheet defined by claim 1 wherein the ratio of ribbon width to thickness is in the range from about 3 to 1 to about 8 to 1.

5. A fabric sheet defined by claim 1 wherein the thermoplastic resin is a mixture of the defined poly-alpha-olefin with up to 15 percent by weight of poly(tetrafluoroethylene).

6. A fabric sheet defined by claim 5 wherein the thermoplastic resin is a mixture of poly(tetrafluoroethylene) and another resin selected from the group consisting of polyethylene, isotactic polypropylene, isotactic poly(l-butene) and isotactic poly(4-methyl pentene).

References Cited UNITED STATES PATENTS 2,717,833 10/1955 Vincent et al. 260-33.6 3,003,304 10/1961 Rasmussen 1 57-157 3,005,795 10/1961 Busse et al. 26045.5 3,081,519 3/1963 Blades et al. 161-173 X 3,099,067 7/ 1963 Merriam et al. 28-82 3,242,246 3/1966 Stand 264-175 X 3,293,844 12/1966 Wininger et al. 57-157 3,294,871 12/ 1966 Schmitt et al. 260-900 3,310,505 3/1967 Parker 260-2.5 3,323,978 6/1967 Rasmussen 161-169 3,351,495 11/1967 Larsen 136-146 3,382,305 5/1968 Breen 264-171 3,407,249 10/ 1968 Landi 264-49 3,485,705 12/ 1969 Harmon 161-59 3,562,374 2/1971 Okamoto et al. 264-128 3,577,308 5/1971 Drunen et al. 161-176 3,132,194 5/ 1964 Edmonds et al 264-37 WILLIAM A. POWELL, Primary Examiner U.S. Cl. X.R. 

